The FastGEO,
“is a library that contains a wide range of highly optimized
computational geometry algorithms and routines for many different
types of geometrical operations such as geometrical primitives and
predicates, hull construction, triangulation, clipping, rotations
and projections.”

SciCraft, a data analysis
framework, with GUI. Works, for example, with Octave and R.

GtkDataBox,
a GTK library for very fast plotting widgets, such as for
oscilloscope data.

OpenDX, "for the
visualization of scientific, engineering and analytical data: Its
open system design is built on a standard interface environments.
And its sophisticated data model provides users with great
flexibility in creating visualizations."

libQGLViewer,
a library to make 3D OpenGL stuff, with already lots of
robotics-related functionality, such as keyframes.

Open Scene
Graph, a "open source high performance 3D graphics toolkit,
used by application developers in fields such as visual simulation,
games, virtual reality, scientific visualization and
modelling."